Microwave isolator



Dec. l, 1959A B. J. DUNCAN 2,915,713

'MICROWAVE IsoLAToR Filed Sept. 13, 1956 2 Sheets-Sheet l INVENTQR ORNEY Dec. 1, 1959 B. J. DUNCAN MICROWAVE IsoLAwoR 2 Sheets-Sheet 2 Filed Sept. 13, 1956 Wag/f vnited States Patent O MICROWAVE ISOLATOR Bobby I. Duncan, Port Washington, N.Y., assignor to Sperry Rand Corporation, a corporation of Delaware Application September 13, 1956, Serial No. `609,587

4 Claims. (Cl. S33-24) This invention relates to microwave transducers employing ferrites, and more particularly to non-reciprocall ferrite transducers, such as microwave isolators.

A non-reciprocal transducer is a device having an energy transfer function depending on the direction of energy passage therethrough. An isolator is a nonreciprocal transducer which freely transfers energy in one direction but prohibits passage of energy in thereverse direction. isolators are employed, for example, in microwave transmission systems to prevent transmitting devices, such as klystrons and magnetrons, from receiving electromagnetic waves reflected from loads, such as antennas. Non-reciprocity is commonly achieved by the employment of ferrite members for interacting with electromagnetic waves. When a ferrite member is placed in the path of a traveling wave and a biasing magnetic eld is applied thereto, the response of the ferrite member will depend on the direction of propagation of the wave.

For example, a ferrite rod magnetically biased in a direction parallel to its axis will rotate the plane of polarization of a linearly polarized wave, the direction of rotation of the plane Vof polarization being dependent on the direction of wave propagation. lf the plane of polarization of a wave traveling toward the load, known as the forward wave, is rotated through 45 a wave reected from the load, known as the backward Wave, will have its plane of polarization rotatedby the same amount and in the same direction with respect to the applied magnetic field direction. The net rotation for the two passes through the ferrite rod is 90. A plane resistive sheet placed at the end of the ferrite rod at which the forward Wave enters is oriented perpendicularly to the plane of polarization of the forward wave will not affect the wave. However, the backward wave, which will have its plane ofpolarization rotated so as to be parallel with the plane of the resistive sheet when it leaves the ferrite rod, will be absorbed in the sheet. A device operating in this manner is known as a Faraday rotation isolator.

Practical Faraday rotation isolators employ a ferrite rod coaxially disposed within a section of circular waveguide. In prior art isolators of this type the ferrite rod has been supported Within the circular waveguide section by thin annular dielectric spacers or by hollow tubes of` foam-like dielectric materiah As these support members are poor heat conductors, no path exists to transfer heat effectively from the ferrite rod to the metal wall of the waveguide section. Thus, in these prior art isolators the ferrite element tends to overheat when operated at high power levels and thereby to become erratic and ineffective.

It is therefore the principal object ofthis invention to provide a Faraday rotation isolator operable at high levels of microwave power. 1

It is a further object of this invention to provide an improved microwave isolator.

It is a further object of this invention to provide a Y, ferrite microwave transducer ,capable of handling highl levels of microwave power.

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It is a further object of this invention to provide a waveguide ferrite isolator in which heat is readily transferred from the ferrite member to the metal wall of the waveguide.

It is a further object of this invention to provide a support means for the ferrite member of a Faraday rotation isolator, said support means also serving to transfer heat from the ferrite member to the waveguide wall, and to absorb the backward wave.

In accordance with the present invention a metal iin extending radially between the ferrite member and the waveguide inner surface serves to effectively transfer heat from the ferrite member to the waveguide wall. A ferrite rod is disposed coaxially within a circular waveguide section. The rod is supported by the metal iin extending radially between the outer surface of the rod and the inside surface of the waveguide section. The iin is twisted about the waveguide axis, the rate of twist and the direction thereof being adjusted so that the fin is everywhere perpendicular to the plane of polarization of the forward dominant mode wave. On the other hand, the plane of polarization of the backward dominant mode wave is rotated to be parallel with the iin and may be absorbed Vif a glossy material is coated on the n. The metal fin serves to support the ferrite member within the waveguide section, to absorb the backward wave, and to conduct heat away from the member.

This invention will be described with reference to the following drawings; wherein Fig. 1 is an elevational view of the preferred embodiment of the isolator of this invention;

Fig. 2 is a perspective view, partly in cross-section, of

the isolator of Fig. l;

Fig. 3v is a series of drawings of the electric eld configurations at various points in the isolator of Fig. l;

Fig. 4 is an alternative embodiment of the isolator of Fig. 1.

To clarify the ensuing explanation of the operation of this device, the following definitions are adopted:

Positive circularly polarized wave: A wave rotating in the direction of the positive electric current which creates a steady longitudinal magnetic iield. y

Negative circularly polarized wave: A Wave rotating in the direction opposite the positive electric current which creates a steady longitudinal magnetic field;

Clockwise rotations-Rotation in a clockwise manner when viewed in the direction of propagation of the wave.

Counter-clockwise rotation: Rotation in a counterclockwise manner when viewed in the direction of propagation of the wave.

Ferrites can be described as polycrystalline materials, of spinel structure which are formed at high temperature by the solid-phase reactions of iron oxide and one or more divalent metallic oxides. By varying the ingredients and the processing techniques, wide ranges in the general properties of ferrites can be obtained. Ferrites in their simplest form correspond to the general chemical formula XOFe2O3, where X represents the divalent metal. Ferrites represented by the above general formula fall into two main classes; those which are ferromagnetic and those which are not. Whether a ferrite falls into one or the other of these classes depends on the divalent metallic oxide used. For example, those ferrites in which X is magnesium, copper, manganese, lithium, nickel, lead, iron, calcium, or cobalt, are ferromagnetic. The ferromagnetic ferrites are ceramic-like materials characterized by low conductivity, low losses, and high permittivity.

It is well known that the R.F. permeability of a saturated ferromagnetic material is not a scalar quantity, but instead the alternating iiux densi-ty in the medium is related to the alternating eld by a tensor permeability. 'Ihe tensor components of the permeability are complex quantities. This unique tensor permeability is the property of ferrites that makes them useful in the microwave art. Thus, circularly polarized waves of opposite sense of rotation encounterdifferent propagation constants in a saturated ferrite member because of this tensor permeability.

yConsider a plane linearly polarized Wave incident on a lossless ferrite member that is subjected to a steady biasing magnetic eld directed parallel to the direction of travel of the wave. A plane linearly polarized wave can be represented by two circularly polarized wave components in phase with respect tothe linearly polarized wave. As the wave passes through the ferrite member the two circularly polarized components encounter different phase constants and they emerge from the ferrite -member having a different phase relationship to each other. The differential phasey shift encountered by the circularly polarized wave components gives rise to rotationv of the plane of polarization of the linearly polarized wave. This wave rotation is known as Faraday rotation. The direction of the rotation of the wave is in the positive sense with respect to the biasing magnetic field. The amount of the rotation is directly proportional to the extent of the ferrite member in the direction of propagation of the incident wave.

Faraday rotation by ferrite members is a non-reciprocal phenomenon because the direction of rotation of the plane of polarization depends only on the direction of Ithe vstatic magnetic field. Relative to an observer looking along the direction of the biasing magnetic field, the direction of rotation of the linearly polarized Wave will be the same whether the wave is traveling toward or away from him; that is, if the plane of polarization of a wave passing through the ferrite member in one direction is rotated a certain amount,` the wave passing through in the opposite direction will have. its plane of polarization rotated by the same amount in the same direction. `Therefore, the net rotation for a wave passing through the ferrite in one direction and then in the other direction is twice the one-pass rotation. This principle is employed in Faraday rotation isolators, such as the isolator of this invention.

ln Figs. l and 2 a circular waveguide section 10 con stitutes `the outer shell of the Faraday rotation isolator of this invention. A pair of circular-to-rectangular waveguide transitions 11 and 12 connect circular waveguide section at each end thereof to respective rectangular waveguide sections 13 and 14. Rectangular waveguide sections 13 and 14 are oriented with respect to each other so that the planes of their respective broad walls intersect at an angle of 45. A cylindrical ferrite rod 16 is disposed within waveguide section 10 concentric with the longitudinal axis thereof. A pair of resistance'cards 17 and 13 are disposed in respective transitions 11 and 12 with their planes parallel to the broad walls of respective rectangular waveguide sections 13 and 14. Means, such as solenoid 19, is provided for immersing ferrite rod 16 in a steady biasing magnetic field, B, whose vector direction is parallel to the axis of waveguide section 10. A thin metallic iin 20 extends between ferrite rod 16 and waveguide section 10, the geometrical configuration of said 1in in a plane perpendicular to the axis of the circular wave guide section being thin rectangles including a.waveguide diameter. Fin 20 supports rod 16 within waveguide sectiont1tl. Finlt) is uniformly twisted aboutan axis coincident with the axis of waveguide section 10. The total twist of the iin 20 is 45. The end of iin .20 nearest rectangular waveguide section 13 is parallel to the broad walls thereof. The end of fin 20 nearest rectangular waveguide 14 is parallel to the broad walls thereof. Fin120 is twisted smoothly and uniformly between its two ends. A lossy material, such as aquadag, may be placed as a coating 21 on .the faces `.of fin 20.

In describing the operation ,of this invention, 'reference will be made to the electric eld configuration drawings of -Fig. 3 and` to'vtheirlocations `in the isolator of Fig. l. The forward wavewill be considered to be one traveling from left to right in Fig. l. The eld congurations in Fig. 3 are drawing looking toward the right in Fig. l. A forward wave in the dominant mode is launched in waveguide section 13, this wave being shown in Fig. 3a, taken at section A-A. The polarization of this wave is perpendicular tothe broad walls of waveguide section 13. This wave travels toward the right `throngh'transition 11 and enters waveguide section=10 in the dominant circular f waveguide mode, shown in Fig. 3b at section B--B. ,Resistance kcard 17 will suppress any cross-polarized components of the wave as it travels toward the right. 'lhisi-insuresl that the direction of polarization of the wave will be perpendicular to the extended broad walls of waveguide section 13 and to the left end of iin 20 as the wave enters circular waveguide section 10. The waveA then encounters n 219, but is substantially unaffected thereby, since the wave -is polarized perpendicularly to the surfaces of the'fiin,-as shown in Fig. 3c at section C-C. The-wave continues to travel toward the right, passing through ferrite-rod 16. The magnitude of the magnetic bias applied to ferrite rod 16 is that necessary to rotate the plane of polarization of the forward wave through a total clockwise angle of 45 as the wave travels to the right through waveguide section 10. The rotation of thewaveV as it travels toward the right is shown successively'in Figs. 3d, 3eand 3f, taken at respective sections D-D,1EE, and F-F. vThe rotation that the wave experiences is uniform and directly proportional to the length ofztheferriteirod. Fin Ztl is twisted to correspond withxthe rotation of theV forward wave. Thus, with the plane of vpolarization of the wave rotated k degrees per unitferrite. length, the tin will be disposed in the geometricalsurface defined .by the cylindrical coordinate equation =kz, where is the angular orientation of the surfaceand zisthe distancefalong the axis of waveguide section 10. Hence, tin 20 is everywhere perpendicular to the direction ofV polarization of the wave and .does not affect it. Theforward-wave leaves waveguide section 10 (Fig. 3 ,section G-.G),.rotated 45 clockwise with respect -to `theorientation withfwhich it entered the waveguide section. The wave4 travels through transition 12 and enters rectangular waveguide section 14as shown in Fig. 3h at section H-H. Resistance card 18 removes any cross-polarized components .of the wave.

Consider now the action of the isolator on a reflected or backward wave traveling toward the left in Fig. l, The vwave enters the isolator from waveguide sectionlt, passes through transition-12 and enters circular waveguidesection 10 oriented perpendicularly to the extended broad walls of waveguide section 14l,.asy shown in Fig. 31' at section F-F. As the wavepasses toward the left its plane of polarization is rotated in a clockwise manner, as shown in Figs. 3j and 3k at respective sections E-E and D-D. As explainedpreviously the absolute ldirection of rotation imparted to the wave is independent of its direction of travel .and depends only on the direction of the biasing magnetic eld. LBoth the-forward wave and backward wave are rotated -in the positiversense with respect to the magnetic feld, B. i As seen in Figs. 3 j and 3k, the field is no longer maintained perpendicular to the iin, but insteadhas a component parallel thereto. This component tends to induce currents in the fin and is absorbed inthe lossy coating 21 on the faces of the fin. Thus, the forward wave is passed without substantial attenuation,:whereas` the `'backward wave is dissipated in iin .20 and the lossy coatingrt21, and `thedevice acts as an isolator. As fin 20 extends betweenferrite rod 16 and the. inner surface of waveguide section. 11D, itv aifords a ready path for the transfer .of heat from the ferrite member and the .dissipative coatingtothe circular waveguide Wall. Consequently, n.20 withy its lossy coating 21 performs the functions of supporting ferrite rod 16, of attenuating the backward wave and of conducting heat to the waveguide wall.

In an alternative embodiment of this invention, shown in Fig. 4, a fin 30 is employed, which is of hollow structure. A pair of conduits 31 and 32 are connected to the fin through the walls of circular waveguide section near the opposite extremities of the n. A suitable coolant may be circulated through the fin by means of conduits 31 and 32 to more effectively cool the structure.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

1. A microwave isolator comprising a section of metallic circular waveguide, a cylindrical ferrite rod coaxially disposed within said waveguide section, means for immersing said ferrite rod in a magnetic field directed along the longitudinal axis of said waveguide, said rod when immersed in said magnetic field being adapted to effect Faraday rotation of a linearly polarized electromagnetic wave of a given frequency passing therethrough,

the Faraday rotation per unit length being equal to a pre determined value, a metallic n disposed within said waveguide section and twisted about an axis coincident with the waveguide longitudinal axis, said n having a rate of twist per unit length equal to said predetermined value, said fin having an elongated aperture disposed parallel to the waveguide longitudinal axis, said ferrite rod being disposed within said aperture, and material dissipative to electromagnetic waves disposed on the faces of said fin, the outer edges of said fin being affixed substantially along their entire lengths to the inner surface of said waveguide and the ferrite rod being atiixed to said fin substantially along the entire lengths of the edges of said aperture, whereby heat conductive paths are provided between said ferrite rod and said waveguide.

2. An isolator as in claim l wherein said fin is of hollow cross section and adapted to receive and emit a circulating coolant.

3. A microwave isolator comprising a section of metallic circular waveguide adapted to propagate linearly polarized electromagnetic waves at a given frequency, a ferrite rod coaxially disposed within said waveguide section, means for immersing said ferrite rod in a magnetic eld directed along the longitudinal axis of said waveguide section, said ferriterod when immersed in said magnetic field being adapted to rotate the plane of polarization of electromagnetic waves at the given frequency by a given angle per unit length, a pair of metallic heat conductive fins disposed radially opposite each other between said rod and the waveguide inner surface, said iins extending throughout the length of said ferrite rod and being twisted at said given angle per unit length about the longitudinal axis of said waveguide, and material dissipative to electromagnetic waves disposed on the faces of said fins, said fins being afiixed substantially along the entire lengths of their outer edges to the waveguide inner surface and substantially along the entire lengths of their inner edges to the surface of the ferrite rod, whereby heat transfer paths are provided between said ferrite rod and the wall of said waveguide.

4. A microwave isolator comprising a section of metallic circular waveguide adapted to propagate linearly polarized electromagnetic wave at a given frequency, a ferrite rod coaxially disposed within said waveguide section, means for immersing said ferrite rod in a magnetic field directed along the longitudinal axis of said waveguide section, said ferrite rod when immersed in said magnetic field being adapted to rotate the plane of polarization of electromagnetic waves at the given frequency by a given angle per unit length, heat conductive means for supporting said ferrite rod coaxially within said waveguide section and for providing good heat transfer paths between said ferrite rod and said waveguide, said heat conductive means comprising metallic iins disposed radially opposite each other between said rod and said waveguide and extending throughout the length of said ferrite rod, said fins being twisted at said given angle per unit length about the longitudinal axis of said waveguide and being affixed substantially along the entire lengths of their outer edges to the inner wall of said waveguide and substantially along the entire lengths of their inner edges to the ferrite rod, and material dissipative to electromagnetic Waves disposed on the faces of said fins.

References Cited in the tile of this patent UNITED STATES PATENTS 2,628,278- Zaleski Feb. 10, 1953 2,748,353 Hogan May 29, 1956 2,752,572 Bird et al. June 26, 1956 2,802,184 Fox Aug. 6, 1957 2,809,354 Allen Oct. 8, 1957 2,850,702 White Sept. 2, 1958 

